JP2006114080A - Wavelength plate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wavelength plate functioning as a quarter wave plate over a wide band of 400 to 780 nm to be a wavelength band required for an optical pickup corresponding to not only optical disks of a CD and DVD but also a blue laser disk. <P>SOLUTION: The wavelength plate 4 is formed by laminating a first wavelength plate 5, a second wavelength plate 6 and a third wavelength plate 7 at prescribed in-plane azimuthal angles, and as substrate materials of the wavelength plates, a quartz crystal is used for the first and the second wavelength plates 5 and 6, and lithium niobate is used for the third wavelength plate 7. The first wavelength plate 5 has a phase difference Γ1=180 deg. (λ=600 nm) and an in-plane azimuthal angle θ1=11.5 deg., the second wavelength plate 6 has a phase difference Γ2=195 deg. (λ=600 nm) and an in-plane azimuthal angle θ2=56.1 deg. and the third wavelength plate 7 has a phase difference Γ3=103 deg. (λ600 nm) and an in-plane azimuthal angle θ3=-45.0 deg. as each parameter of the wavelength plate 4. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a wave plate, and more particularly, to a wave plate used for an optical pickup compatible with a blue laser disk as well as an optical disk such as a CD or a DVD.

An optical pickup is used when information is recorded on or reproduced from an optical disc such as a CD or a DVD, and a quarter-wave plate is used for the optical pickup in order to make the laser light applied to the optical disc circularly polarized. .
The quarter-wave plate is an optical element that performs phase modulation using a birefringent material, and has a function of converting incident linearly polarized light into circularly polarized light. On the other hand, as is well known, the phase difference between the light incident on the wave plate and the emitted light is a function of the wavelength, so even if it functions as a quarter wave plate for light of a specific wavelength. If the wavelength of the incident light is different from that, it has a wavelength dependency that the phase difference changes, and therefore the phase difference cannot be maintained at ¼ wavelength.

Therefore, a broadband quarter-wave plate has been proposed in Japanese Patent Laid-Open No. 10-68816.
FIG. 11 shows an example of the external structure of a conventional broadband quarter-wave plate disclosed in Japanese Patent Application Laid-Open No. 10-68816, (a) shows a front view (transmission surface), and (b) shows a side view. Show. This broadband quarter wave plate is formed by laminating two wave plates. A quarter wave plate and a half wave plate are bonded together at a predetermined angle to obtain a desired quarter wave plate. As the performance has been obtained. As shown in FIG. 11, the broadband quarter wave plate 1 includes a first wave plate 2 and a second wave plate 3 that are referred to as an optical axis azimuth angle (hereinafter referred to as an in-plane azimuth angle) of the first wave plate 2. ) Θ1 is set to 15 deg, and the in-plane azimuth angle θ2 of the second wave plate 3 is set to 75 deg.

Therefore, a conventional method for designing a broadband quarter-wave plate disclosed in Japanese Patent Laid-Open No. 10-68816 will be described below.
The phase difference of the first wave plate 2 is 180 deg, the in-plane azimuth angle is θ1, the phase difference of the second wave plate 3 is 90 deg, the in-plane azimuth angle is θ2, and explanation is made using the Poincare sphere shown in FIG. To do.
FIG. 12 shows a conventional Poincare sphere of a quarter wave plate. In FIG. 12, when incident light is incident on a predetermined position P0 on the equator of the Poincare sphere, this incident light is modulated by the first wave plate 2 and reaches P1, and further by the second wave plate 3. When the light reaches the pole P2 of the Poincare sphere after being modulated, the light beam emitted from the quarter-wave plate 1 becomes circularly polarized light. Here, in order for P2 to be the pole of the Poincare sphere, it is desirable that θ1 and θ2 satisfy the following relational expression.
θ2 = 2θ1 + 45 (1)

  When the wavelength of the incident light changes between λ1 and λ2, the phase difference between the first wave plate 2 and the second wave plate 3 changes from 180 deg and 90 deg, respectively. The amount of change in the first wave plate 2 is ΔΓ1, and the amount of change in the second wave plate 3 is ΔΓ2. Here, P1 on the Poincare sphere, where the incident light is modulated by the first wave plate 2 and reached, changes the phase amount of the wave plate due to the change of the wavelength of the incident light, and shifts to P1 ′. Assuming that the change amounts ΔΓ1 and ΔΓ2 are the same line segment on the spherical surface connecting P1 and P1 ′ on the Poincare sphere, P2 can always reach the pole of the Poincare sphere.

Therefore, if P1 and P1 ′ are approximately connected by a straight line and the relationship between ΔΓ1, ΔΓ2, and θ1 is expressed using the cosine theorem,
cosΔΓ2 = 1−2 (1-2cos2θ1) (1-cosΔΓ1) (2)
It becomes. If the first wave plate 2 and the second wave plate 3 are made of the same wavelength dispersion material, the phase differences are 180 deg and 90 deg.
ΔΓ1 = 2ΔΓ2 (3)
Satisfy the relationship. Substituting this into equations (1) and (2) gives the following values for θ1 and θ2.
θ1 ≒ 15deg, θ2 ≒ 75deg

From the above results, the following conditions are essential for the function of the quarter-wave plate 1 as a broadband quarter-wave plate.
First wave plate 2 phase difference 180 deg
In-plane azimuth 15 deg
Second wave plate 3 phase difference 90 deg
In-plane azimuth 75deg
However, since the above condition includes approximation, simulation is performed by using a Mueller matrix, Jones matrix, etc. so as to obtain optimum characteristics for actual use.

FIG. 13 shows an example of ellipticity characteristics of a conventional broadband quarter-wave plate. FIG. 13 is a graph showing the result of optimization so that the ellipticity approaches 1 most in a predetermined wavelength range, and characteristic example 1 shows an example optimized in the wavelength range of 600 to 850 nm. Characteristic example 2 shows an example of optimization in the wavelength range of 400 to 500 nm, and it can be seen that each of them functions as a quarter-wave plate in a predetermined wavelength range.
JP-A-10-68816 JP 2003-248198 A

  The optical material of the wave plate generally has a characteristic that the shorter the wavelength, the larger the change of the refractive index with respect to the wavelength. The shorter the wavelength, the narrower the wavelength range that functions as a quarter wave plate. On the other hand, in recent years, in addition to conventional optical disks such as CD and DVD, an optical disk called a blue laser disk having a larger capacity has been put into practical use. Is also required to respond to. The blue laser disk uses a laser beam with a wavelength of around 400 nm, and in order to be compatible with an optical pickup using a conventional 650 nm or 780 nm laser beam, a quarter wavelength is used. The wavelength band necessary for the plate is a wide band of 400 nm to 780 nm. As can be seen from the characteristic example shown in FIG. 13, even if the conventional broadband wave plate can optimize the wavelength range of 400 to 550 nm, it is difficult to optimize the wavelength range of 550 to 780 nm.

  The present invention has been made in order to solve the above-described problems, and has a wavelength band of 400 nm to 780 nm, which is a wavelength band necessary for an optical pickup corresponding to a blue laser disk as well as a CD or DVD optical disk. An object of the present invention is to provide a wave plate that functions as a quarter wave plate.

In order to achieve the above object, the wave plate according to the present invention has the following configuration.
The wave plate according to claim 1 is formed by laminating the first wave plate, the second wave plate, and the third wave plate in order after setting the in-plane azimuth angle of the crystal optical axis to a predetermined angle. In the wave plate to be configured, the incident position on the Poincare sphere of the incident polarized light incident on the first wave plate is P0, the position on the Poincare sphere of the light modulated in the first wave plate is P1, the second When the position on the Poincare sphere of the light modulated on the wave plate of P2 is P2, the triangle surrounded by the three positions P0, P1, and P2 at a wavelength of 400 to 780 nm, and the others of 400 to 780 nm The triangle surrounded by the three positions P0, P1, and P2 at the same wavelength is similar to each other, and the position P3 on the Poincare sphere of light modulated by the third waveplate is always the coordinates of the north pole of the Poincare sphere Or Configured to reach the coordinates of the Antarctic.

The wave plate according to claim 2 is the wave plate according to claim 1, wherein the phase difference of the first wave plate is Γ1, the in-plane azimuth angle is θ1, and the phase difference of the second wave plate is Γ2. When the in-plane azimuth is θ2, the phase difference of the third wave plate is Γ3, and the in-plane azimuth is θ3, each parameter is
Phase difference Γ1 = 180 deg, in-plane azimuth angle θ1 = 11.5 deg,
Phase difference Γ2 = 195 deg, in-plane azimuth angle θ2 = 56.1 deg,
Phase difference Γ3 = 103 deg, in-plane azimuth angle θ3 = −45.0 deg
To be configured.

The wideband phase compensation wave plate according to claim 3 is the wave plate according to claim 1 or 2, wherein the substrate material of the first wave plate and the second wave plate is quartz, and the third wavelength. The substrate material of the plate is configured as lithium diobate.

  According to the first to third aspects of the present invention, the broadband phase compensation wave plate is formed by laminating three wave plates having a predetermined phase difference and an in-plane azimuth angle, and the first wave plate and the second wave plate are formed. By using quartz as the substrate material and lithium niobate as the substrate material of the third wave plate, it functions as a quarter wave plate in a wide wavelength range of 400 to 780 nm, and CD In addition to DVD optical disks, it has optical characteristics necessary in the wavelength band necessary for optical pickups that are compatible with blue laser disks, and exhibits a great effect in constructing optical pickups.

Hereinafter, the present invention will be described in detail based on illustrated embodiments.
The wavelength plate according to the present invention functions as a quarter wavelength plate over a wide band of 400 nm to 780 nm which is a wavelength band necessary for an optical pickup compatible with a blue laser disk as well as an optical disk of CD and DVD. Three wave plates having a predetermined phase difference and in-plane azimuth are laminated.

  FIG. 1 shows an external structural view showing an embodiment of a wave plate according to the present invention, (a) shows a front view (transmission surface), and (b) shows a side view. The wave plate 4 is obtained by laminating a first wave plate 5, a second wave plate 6, and a third wave plate 7 with a predetermined in-plane azimuth angle. However, in the wide wavelength range of 400 to 780 nm, the parameters are set as follows to function as a quarter-wave plate. Further, the optical axis of each wave plate is parallel to the light incident surface as shown in FIG. As will be described later, as the substrate material of the wave plate, the first wave plate 5 and the second wave plate 6 use quartz, and the third wave plate 7 uses lithium niobate. .

First wave plate 5 Phase difference Γ1 180 deg (λ = 600 nm)
In-plane azimuth angle θ1 11.5 deg
Second wave plate 6 Phase difference Γ2 195 deg (λ = 600 nm)
In-plane azimuth angle θ2 56.1 deg
Third wave plate 7 retardation Γ3 103 deg (λ = 600 nm)
In-plane azimuth angle θ3 -45.0deg

Next, a method for designing a wave plate according to the present invention will be described using a Poincare sphere.
FIG. 2 is a view showing a Poincare sphere of a wave plate according to the present invention. 2A shows a view from the S3 axis (north pole side), and FIGS. 2B to 2D show the optical axis R1 of the first wave plate 5 and the second wave plate 6, respectively. FIG. 2A is developed by trigonometry in each direction of the optical axis R2 and the optical axis R3 of the third wave plate 7. FIG.
In FIG. 2, it is assumed that the coordinate (1, 0, 0) on the equator is P0, and incident polarized light is incident on this coordinate, the position modulated in the first wave plate 5 is P1, and the second wave plate 6 The position modulated in step P2 is P2, and the position modulated in the third wave plate 7 is P3.

  At this time, the first wave plate 5 and the second wave plate so that P3 reaches the coordinates of the north pole (0, 0, 1) or the coordinates of the south pole (0, 0, -1). If the parameters of 6 and the third wave plate 7 are set, the light beam that has passed through the three wave plates becomes circularly polarized light.

  In addition, in order to function as a quarter wavelength plate in a wide wavelength range of 400 to 780 nm, the wavelength plate of the wavelength plate is changed by changing the wavelength of incident light, the first wavelength plate 5, the second wavelength plate 6, Even if the respective phase differences Γ1, Γ2, and Γ3 of the third wave plate 7 change, P3 that is a position modulated in the third wave plate 7 is the coordinates of the north and south poles (0, 0, 1) or (0, 0, -1) may be reached.

  For this purpose, the respective parameters of the first wave plate 5, the second wave plate 6, and the third wave plate 7 are set to optimum values, and the incident polarized light is incident at a wavelength of 400 to 780 nm. A triangle surrounded by three positions: a position P0, a position P1 modulated in the first wave plate 5 and a position P2 modulated in the second wave plate 6, and P0 and P1 at other wavelengths of 400 to 780 nm. , The position P3 modulated in the third wave plate 7 is always the coordinates of the north pole (0, 0, 1) or the south pole. (0, 0, −1), which is the coordinates of

Therefore, a method for designing a wave plate according to the present invention will be described below using mathematical expressions.
When the Poincare sphere radius is 1, P0-P1 (hereinafter referred to as L1) which is one side of the triangle can be expressed by the following equation using the parameters of each wave plate.
L1 = sin2θ1 (1−cos Γ1) (4)

On the other hand, P1-P2 (hereinafter referred to as L2) which is one side of the triangle can be expressed by the following equation using the parameters of each wave plate.
L2 = cos2θ2 · {cos (cosα−cos (Γ2−α))} (5)
However, α = sin ⁻¹ (sin2θ1 · sinΓ1) / cos2θ2 (6)

If the angle formed by the sides L1 and L2 of the triangle is b, b can be expressed by the following equation.
∠b = 180−2θ1 + 2θ2 (7)
Here, using the cosine theorem, P2-P0 (hereinafter referred to as L3) which is one side of the triangle can be expressed by the following equation.
L3 ² = L1 ² + L2 ² −L1 · L2 · cos2b (8)

Therefore, when L3 is obtained by substituting equations (4), (5), (6), and (7) into equation (8), it can be expressed by the following equation.
L3 ² = sin ² 2θ1 · (1-cos Γ1) ²
+ Cos ² 2θ2 · {cos α−cos (Γ2 + α)} ²
+ 2sin2θ1 · (1-cosΓ1)
Cos 2θ2 {cos α-cos (Γ2 + α)} cos (2θ1-2θ2) (9)

On the other hand, L3, which is one side of the triangle, can be expressed using Γ3, which is a parameter of the third wave plate 7, and can be expressed by the following equation when this is L3 ′.
L3 ′ = 1 + sin Γ3 (10)

  From the above results, L3 represented by the equation (9) obtained from the parameters of the first wave plate 5 and the second wave plate 6 and the equation (10) obtained from the parameters of the third wave plate 7 are obtained. If the represented L3 ′ is equal, the triangle surrounded by the sides L1, L2, and L3 has a similar relationship in a wide wavelength range of 400 to 780 nm.

here,
L = L3-L3 ′ (11)
If the parameters of the first wave plate 5, the second wave plate 6, and the third wave plate 7 are set so that L≈0, the first wave plate 5 and the second wave plate 6 can be canceled by the phase difference change due to the wavelength in the third wave plate 7, and the position P3 that is always modulated in the third wave plate 7 is the coordinates of the north pole. A certain (0, 0, 1) can be reached. Therefore, the wave plate according to the present invention functions as a quarter wave plate in a wide wavelength range of 400 to 780 nm, and converts incident light into circularly polarized light.

Next, using the design method as described above, simulation was performed by changing the parameters, and the results were summarized in a graph.
Therefore, as a first design example, the parameters of each wave plate are set so that the wavelength of the light beam is L≈0 around 400 nm.
FIG. 3 is a graph showing the wavelength characteristics of L3, L3 ′, and L, which are one side of the triangle, in the first design example of the wave plate according to the present invention. The parameters of each wave plate at that time are as follows.

First wave plate 9 phase difference Γ1 220 deg (λ = 405 nm)
In-plane azimuth angle θ1 -13deg
Second wave plate 10 phase difference Γ2 210 deg (λ = 405 nm)
In-plane azimuth angle θ2-53deg
Third wave plate 11 phase difference Γ3 100 deg (λ = 405 nm)
In-plane azimuth angle θ3 45deg

FIG. 4 shows an external structural view showing a first design example of the wave plate according to the present invention, (a) shows a front view (transmission surface), and (b) shows a side view. The wave plate 8 is obtained by laminating a first wave plate 9, a second wave plate 10, and a third wave plate 11 with a predetermined in-plane azimuth angle.

FIG. 5 is a graph showing the wavelength characteristics of ellipticity when parameters are set according to the first design example in the wave plate according to the present invention. FIG. 5 also shows the characteristic example 1 and the characteristic example 2 showing the ellipticity of the conventional broadband quarter-wave plate in order to compare the characteristics.
The wavelength characteristic of the ellipticity according to the first design example shown in the graph has a band of about 350 nm, and the wavelength characteristic of the conventional broadband quarter-wave plate has a band of about 200 nm. Considering this, it can be seen that it functions as a quarter-wave plate in a fairly wide band, but it is a wavelength band necessary for an optical pickup corresponding to a blue laser disk as well as a CD and DVD optical disk. It has not yet functioned as a quarter-wave plate over a wide band of 780 nm.

Then, next, as a second design example, a case where it functions as a broadband wave plate in a higher wavelength band than the first design example is studied, and each wavelength is set so that the wavelength of the light beam becomes L≈0 near 650 nm. The board parameters were set.
FIG. 6 is a graph showing the wavelength characteristics of L3, L3 ′, and L, which are one side of the triangle, in the second design example of the wave plate according to the present invention. The parameters of each wave plate at that time are as follows.

First wave plate 13 Phase difference Γ1 180 deg (λ = 650 nm)
In-plane azimuth angle θ1 -13deg
Second wave plate 14 phase difference Γ2 180 deg (λ = 650 nm)
In-plane azimuth angle θ2-53deg
Third wave plate 15 phase difference Γ3 90 deg (λ = 650 nm)
In-plane azimuth angle θ3 45deg

  7A and 7B are external structural views showing a second design example of the wave plate according to the present invention, FIG. 7A is a front view (transmission surface), and FIG. 7B is a side view. The wave plate 12 is obtained by laminating a first wave plate 13, a second wave plate 14, and a third wave plate 15 at a predetermined in-plane azimuth angle.

FIG. 8 is a graph showing the wavelength characteristics of ellipticity when parameters are set according to the second design example in the wave plate according to the present invention. In FIG. 8, for comparison, the characteristic example 1 and the characteristic example 2 showing the ellipticity of the conventional broadband quarter wave plate are also shown.
The wavelength characteristic of the ellipticity of the second design example shown in the graph has a band of about 350 nm as in the first design example described above, and the wavelength characteristic of the conventional wave plate is about 200 nm. Although it can be seen that it functions as a quarter-wave plate in a fairly wide band, it is necessary for optical pickups compatible with CD and DVD optical discs as well as blue laser discs. It has not yet functioned as a quarter-wave plate over a wide wavelength range of 400 nm to 780 nm.

  On the other hand, in the first design example and the second design example as described above, quartz is used as the substrate material for the first wave plate, the second wave plate, and the third wave plate, respectively. Although the laminated wave plate was configured, the applicant of the present application has made various other materials without limiting the birefringent material used as the substrate material of the wave plate to quartz in order to further increase the bandwidth. The case of using was examined.

  FIG. 9 is a graph showing the birefringent wavelength dispersion of the main birefringent material. FIG. 9 shows that the birefringence wavelength dispersion of quartz, BBO, and calcite is small, and the birefringence wavelength dispersion of lithium niobate and YVO4 is large. Therefore, the inventor of the present application performs a simulation for each of the cases where the materials having the birefringence are used in combination as substrate materials for the first wave plate, the second wave plate, and the third wave plate. A study was made so that the expression (11) becomes L≈0 in the band.

  As a result of the simulation, the inventor of the present application uses a crystal having a small birefringence wavelength dispersion as a substrate material of the first wave plate and the second wave plate, and a large birefringence wavelength dispersion as a substrate material of the third wave plate. It has been found that when lithium diobate is used, the laminated wave plate has a desired broadband. The third design example describes the above-described embodiment. At this time, the parameters of the first wave plate, the second wave plate, and the third wave plate obtained as a result of the simulation are described above. As follows.

First wave plate 5 Phase difference Γ1 180 deg (λ = 600 nm)
In-plane azimuth angle θ1 11.5 deg
Second wave plate 6 Phase difference Γ2 195 deg (λ = 600 nm)
In-plane azimuth angle θ2 56.1 deg
Third wave plate 7 retardation Γ3 103 deg (λ = 600 nm)
In-plane azimuth angle θ3 -45.0deg

  FIG. 10 is a graph showing the wavelength characteristics of the ellipticity of a wave plate having parameters set according to the third design example having a desired band in the wave plate according to the present invention. FIG. 10 also shows the ellipticity of the first design example and the second design example in order to compare the characteristics. As shown in FIG. 10, in the third design example, the wavelength plate has a characteristic band of about 480 nm, a wide band of 400 to 780 nm, and an ellipticity of 0.8 or more. The wave plate characteristics as a four-wave plate were satisfied.

  As described above, in the present invention, the substrate material of the three wave plates constituting the laminated wave plate, the first wave plate and the second wave plate are quartz, and the third wave plate is diobium acid. By using lithium, a broadband wave plate that functions as a quarter wave plate over a wide band of 400 nm to 780 nm, which is a wavelength band necessary for an optical pickup compatible with a blue laser disk as well as an optical disk of CD and DVD. It became possible to provide.

1 is an external structural view showing an embodiment of a wave plate according to the present invention. It is a figure which shows the Poincare sphere of the wave plate concerning this invention. In the first design example of the wave plate according to the present invention, it is a graph showing the wavelength characteristics of L3, L3 'and L which are one side of the triangle described above. 1 is an external structural view showing a first design example of a wave plate according to the present invention. FIG. In the waveplate concerning this invention, it is a graph which shows the wavelength characteristic of the ellipticity at the time of setting a parameter by the 1st design example. It is a graph which shows the wavelength characteristic of L3 and L3 'which are one side of the triangle mentioned above in the 2nd design example of the waveplate concerning this invention. The external appearance structural view which shows the 2nd design example of the wave plate concerning this invention is shown. In the waveplate concerning this invention, it is a graph which shows the wavelength characteristic of the ellipticity at the time of setting a parameter by the 2nd design example. It is a graph which shows the birefringent wavelength dispersion of the material with main birefringence. In the waveplate concerning this invention, it is a graph which shows the wavelength characteristic of the ellipticity of the waveplate which set the parameter by the 3rd design example which has a desired zone | band. An example of the external structure of a conventional broadband quarter-wave plate disclosed in Japanese Patent Laid-Open No. 10-68816 is shown. The Poincare sphere of the conventional quarter wavelength plate is shown. The example of the ellipticity characteristic of the conventional broadband quarter wave plate is shown.

Explanation of symbols

1. Broadband quarter wave plate, 2. First wave plate,
3. Second wave plate, 4. Wave plate,
5. First wave plate, 6. Second wave plate,
7 .... Third wave plate, 8 .... Wave plate,
9. First wave plate, 10. Second wave plate,
11 .... Third wave plate, 12 .... Wave plate,
13 .... first wave plate, 14 .... second wave plate,
15. Third wave plate

Claims (3)

In the wave plate configured by laminating the first wave plate, the second wave plate, and the third wave plate in order after setting the in-plane azimuth angle of the crystal optical axis to a predetermined angle,
The incident position on the Poincare sphere of the incident polarized light incident on the first wave plate is P0, the position on the Poincare sphere of the light modulated on the first wave plate is P1, and is modulated on the second wave plate. When the position of the light on the Poincare sphere is P2, the triangle surrounded by the three positions P0, P1 and P2 at a wavelength of 400 to 780 nm, and P0 and P1 at other wavelengths of 400 to 780 nm , And the triangle surrounded by the three positions P2 are similar to each other, and the position P3 on the Poincare sphere of light modulated in the third wave plate is always the coordinates of the North Pole or the South Pole of the Poincare sphere. A wave plate characterized by being configured to reach The phase difference of the first wave plate is Γ1, the in-plane azimuth angle is θ1, the phase difference of the second wave plate is Γ2, the in-plane azimuth angle is θ2, and the phase difference of the third wave plate is When Γ3 and the in-plane azimuth angle is θ3, each parameter is
Phase difference Γ1 = 180 deg, in-plane azimuth angle θ1 = 11.5 deg,
Phase difference Γ2 = 195 deg, in-plane azimuth angle θ2 = 56.1 deg,
Phase difference Γ3 = 103 deg, in-plane azimuth angle θ3 = −45.0 deg
The wave plate according to claim 1, wherein the wave plate is configured as follows. 3. The wavelength according to claim 1, wherein the substrate material of the first wave plate and the second wave plate is quartz, and the substrate material of the third wave plate is lithium diobate. Board.

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